Fine Woodworking 2007 Building Furniture

added benefit of covering any bruised edges on the mortise that

result from cutting the joint.

Centered tenons are ideal but not always practical—One

engineering principle states that the stress on any part is least

along the centerline or neutral axis. A centered mortise and tenon

is strong because it leaves plenty of wood on both walls (sides) of

the mortise and creates substantial shoulders on the tenons. But a

centered mortise and tenon does not always work visually.

Deciding on the exact placement is a judgment call that var-

ies with each project. The farther to the outside of the leg you

position a mortise, the longer the respective tenon will be. Too

far out, though, and the mortise wall will be more vulnerable to

splitting under stress. To maximize the tenon length in such situ-

ations, I often design the joint so that the tenons meet inside the

leg. I have butted tenons together, but doing so makes one tenon

shorter than the other. I prefer to miter the tenons within the joint

(see drawing, left). This is easy to do, and it can add 15% to 20%

more glue surface and length to the tenons.

Haunched tenon stabilizes the top of the joint

A tenon the full height of the apron affords lots of glue surface and

strength against bending and twisting forces. But there’s a trade-

off: A full-height mortise

weakens the leg, especially

if there are two mortises at

the corner of the leg, and

serious stress on the apron

can more easily split the

top of the leg.

A simple solution is to

shorten the tenon at the

top by^3 ⁄ 4 in. to 1 in. or so

and cut an angled haunch

near the top shoulder. This

design leaves material at

the top of the mortise,

which makes for a stron-

ger joint.

I cut the haunch with

a dovetail saw and then

clean it up with a chisel

(see photos, facing page).

For speed and accuracy, I

mark out the haunch on

the tenon using a template. After cutting the mortise for the tenon,

I use another angled template to guide the chisel as I cut the

haunched area. Because I cut many of my mortises with a router

bit, I keep the top of the mortise below the haunch round for a

small measure of added strength. Also, a small^1 ⁄ 8 -in. shoulder at

the bottom of the apron tenon will hide any small inaccuracies

in cutting the mortise, and it allows for vertical alignment when

the table is assembled.

Adjust the fit and use glue sparingly

The best design and the strongest glue won’t overcome a joint

with carelessly fit shoulders or a sloppy fit between tenon and

mortise. Even when I cut these joints with accurate machine

setups, I still often find it necessary to improve the fit with a few

a s t u r dy l e g - t o - a p ro n j o i n t

Haunched tenon

resists twisting

and avoids splits

at top of leg.

Square pins,

rounded

over to make

dowels,

provide a

mechanical

lock.

Chisel the routed

mortise square at

the bottom.

Square the

openings of

the pin holes

with a chisel.

Locate the tenon near

the outside face of the

apron to maximize its

length.

Leave a small

gap for glue

between

mitered

tenons.

Tenon should be a little

more than one-third the

thickness of the apron.

Length of the tenon

should be three to four

times its thickness.

Leave the leg

long and trim

to size after

mortising.

Offset the

pin holes

to avoid

splitting the

leg.

Small shoulder at the

bottom ensures a

side view clean line at the joint.

Top view

Glue only the top

of the tenon.

wide aprons need a break

A mortise longer than 4 in. or so

can threaten the structural integrity

of a leg. A break in the middle for

a haunched tenon alleviates that

problem but still keeps the apron

from twisting.

Stub

Elongate the bottom hole

to allow movement.

36 F I N E wO O Dw O r kI Ng Photos, except where noted: william Duckworth; drawings: Bob La Pointe

FWSIP08BF_SJ.indd 36 6/6/07 12:28:25 PM